The present invention relates to an active mechanical hydraulic control apparatus. More specifically, the invention relates to a hydraulic actuator for operating, for example, an elevator or rudder flap of an aircraft or other craft in response to an attitude change of the craft. Such positional or attitude changes may be caused by wind or wave forces and cannot be corrected positively by conventional passive dampers.
U.S. Pat. No. 2,087,350 (McCann) issued July 20, 1937 discloses a passive damper which converts shock energy, for example when a vehicle wheel travels through a hole in the road, into hydraulic pressure which in turn is dissipated through throttling valves. Two pistons are moved in response to vehicle body movements caused by shocks effective in one or the other direction. Both pistons are moved by the same lever arm which senses excursions of the vehicle wheel. The pressure building up in one or the other chamber due to the respective piston movement is dissipated through two series connected valves in a respective return flow passage for the hydraulic medium. One valve in each return flow passage is a spring biased pressure responsive valve. The other valve connected in series with the pressure responsive valve in each return flow passage is controlled by the movement of an inertia mass. The arrangement is such that the inertia movement responsive valve is normally open so that for damping of average shocks the pressure responsive valve is effective. The inertia responsive valve becomes only effective in response to heavy shocks. To provide for a smooth vehicle ride the structure of U.S. Pat. No. 2,087,350 requires a dashpot device for the inertia responsive control of the valves. Such a structure is an effective passive damper or shock absorber but it is not suitable for actively or positively actuating, for example, an elevator flap of an aircraft in response to changes in the attitude of the aircraft as a result of wind gusts.
U.S. Pat. No. 1,783,348 (Taylor) issued Dec. 2, 1930 discloses a shimmying damper which is intended to dampen vehicle wheel vibrations without impeding the steering action. Taylor discloses a piston cylinder arrangement in which the piston is normally biased into a central or neutral position by two springs. Such a structure is also not suitable for an effective correction of the attitude of a craft by positively adjusting the position of a flap or the like in response to the excursion of the craft from a predetermined attitude.
Prior art active control devices in the steering system of a craft generally comprises a combination of mechanical acceleration sensors, a valve and a piston operating as an actuator or adjustment member.
The valve controls, in response to sensed accelerations, the oil flow in a piston cylinder control unit or arrangement in such a manner that the vibration amplitudes of a constant reference mass connected to the piston of the arrangement, are substantially reduced. If the mass to be damped is not constant, but rather variable, it is necessary to provide an additional power or follower amplifier also comprising a control valve and a piston cylinder arrangement which is mechanically connected between the piston of the active control unit and the constant reference mass. The accelerations effective on the reference mass cause pressure to be applied to the piston of the control unit in such a way, that, ideally, the effective pressures may be reduced to zero since they balance each other. Thus, the constant reference mass is movable relative to the vehicle and it may be substantially stationary relative to a fictitious reference point on the earth. Due to the mechanical connection with the valve slide of the follower amplifier and due to the hydraulic coupling between the valve slide and the cylinder a relative movement of practically the same amplitude occurs between the mass to be damped and the vehicle.
Similarly constructed active control devices are known as electro-mechanical or electro-hydraulic systems especially used in aircraft engineering. Such systems are used in active control systems of limited authority for compensating aircraft movements resulting from disturbances such as wind gusts causing movements about the roll axis, the pitch axis, and the yaw axis. These prior art control systems comprise electro-mechanical sensors such as gyros, acceleration pick-ups, and include electronic signal processing means as well as electro-hydraulic actuator means with electrical resetting means.
The operability of these systems must be assured at all times because errors and failures cannot be compensated by simply shutting off the defective part. This assured operability must especially be satisfied by the mechanical, electro-mechanical, and electro-hydraulic components of the active regulating or control systems.
In these active prior art systems the operability is frequently impaired at the interface of the signal transmission, that is, between the mechanical-electrical sensors, the electrical signal processing, the electro-hydraulic adjustment drives and the electrical resetting means. A further disadvantage of prior art systems is seen in that they are too heavy and require too much space.